Fuel injection systems for internal combustion engines typically include a mass air flow sensor for sensing air flow into the engine intake manifold. Data from the sensor is used to calculate a cylinder air charge from which the fuel injector pulse width needed to achieve a desired air/fuel ratio is determined. One prior art approach is disclosed in Follmer et al U.S. Pat. No. 5,159,914, assigned to the assignee of the present invention, and incorporated herein by reference. In Follmer et al a hot wire mass airflow sensor provides raw airflow rate data to the computer. The computer corrects the raw data to compensate for airflow leakage in the intake system. The compensated airflow data is used in a manifold filling model that modifies the data to account for the effects of engine load on volumetric efficiency of the engine. The computer calculates the cylinder air charge utilizing the modified data from the manifold filing model. After cylinder air charge is determined, the fuel injector pulse width necessary to achieve a desired air/fuel ratio is determined. Another method of predicting cylinder air charge is disclosed in U.S. Ser. No. 08/016,322 filed Feb. 10, 1993, assigned to the assignee of the present invention, the disclosure of which is incorporated herein by reference.
One measure taken to improve fuel efficiency and reduce harmful emissions in internal combustion engine is to vary the timing or relative phase angle between the camshaft and crankshaft to achieve more optimum engine operation at various speed/load conditions. One approach to phaseshift control is disclosed in Maurer et al U.S. Pat. No. 5,209,202 assigned to the assignee of the present invention and incorporated herein by reference. That phase shift control uses a hydraulic device which rotationally couples a camshaft drive sprocket to a camshaft flange. The engine control computer in the aforementioned patent responds to continuously varying engine operating conditions to control the hydraulic device and set the desired relative phase angle for the condition existing at any point in time. A suitable hydraulic device is disclosed in Schechter et al U.S. Pat. No. 5,117,784 assigned to the assignee of the present invention and incorporated herein by reference.
Volumetric efficiency (Veff) describes how effective an engine is at filling a cylinder and is expressed as a percentage of total cylinder capacity. Because the cylinder does not totally fill up with air and fuel, Veff typically varies from 50% to 85% of the total cylinder capacity. Veff is influenced by such factors as cylinder displacement, piston stroke, air intake geometry, and valve timing. The intake system, in particular the intake valve, restricts the amount of air which an engine can induct. Varying the cam timing in response to continuously varying engine operating conditions as discussed above affects the time when the intake valve opens and closes and thus changes the Veff of the engine. In order to maintain or improve fuel economy gains and the reductions in harmful emissions obtained with variable cam timing, it is desirable that the strategy for calculating cylinder air charge, and thus fuel pulse width, account for the changes in Veff which occur when cam timing is varied.